Communications between a transmitter and receiver generally require some form of synchronization in time and/or frequency before transmissions of messages can be received reliably. In cellular systems, such as Long Term Evolution (LTE), base stations broadcast narrowband synchronization signals regularly in time. These synchronization signals allow wireless devices accessing the system to perform an initial cell search. For example, wireless devices may go through a synchronization procedure that includes finding carrier frequency, time reference instants, and cell identity. A LTE wireless device that has performed initial cell search and identified the cell identity can then complete the initial synchronization in downlink by making a fine synchronization on cell specific reference signals that are transmitted over the system bandwidth and more frequently in time than the synchronization signals. The wireless device connects to the network via a random access procedure in which uplink time synchronization will be established and communications between the device and the base station can begin. Due to oscillator drifting at both transmitter and receiver sides, the wireless device needs to regularly perform fine frequency synchronization in downlink during the communications with the base station.
A lean frame structure design for NX without cell-specific reference signals (CRS) has been proposed where instead reference signals required for fine synchronization and demodulation of a downlink (DL) physical data channel (PDCH) are embedded into the PDCH transmission. FIG. 1 illustrates the DL transmissions of PDCH and associated physical downlink control channel (PDCCH), carrying an assignment or a grant. More specifically, FIG. 1 illustrates that the first Orthogonal Frequency Division Multiplexing (OFDM) symbol of a subframe contains PDCCH and following OFDM symbols contain PDCHs.
As also illustrated in FIG. 1, transmissions of PDCH may span over multiple subframes in the case of subframe aggregation or be confined to one subframe. A wireless device, which may also be referred to as user equipment (UE), detects PDCCH addressed to the UE and derives from the scheduling information PDCH related information. A UE is not aware of PDCCH transmissions to other UEs where a PDCCH to one particular user is carried on a subset of OFDM subcarriers. The mapping of PDCCH can either be distributed or localized with latter being illustrated in FIG. 1. The number of OFDM symbols within a subframe is a system design parameter and may very well be larger than the 4 used in the depicted example.
In the illustrated example, PDCCH and PDCH have their own reference signals for demodulation which mainly refer to Demodulation Reference Signals (DMRS) but could potentially also refer to other types of reference signals as will be discussed herein. The DMRS should be transmitted early in the subframe to enable the receiver to perform early channel estimation and by that reduce receiver processing time.
In the context of NX, time-synchronization is done using a first reference signal (e g., a Time Synchronization Signal (TSS)) and coarse-frequency-sync using the same first reference signal or a second signal (e.g., Frequency Synchronization Signal). One may observe that these signals are not intended to provide a very accurate synchronization, neither in time nor in frequency. The time-error can be handled by the cyclic-prefix in an OFDM system and the frequency error by having sufficient sub-carrier spacing. However, in order to not limit the performance of higher rank transmissions of PDCH in conjunction with higher modulation (such as 64 and 256 QAM) schemes, better frequency-synchronization is needed. State of the art solutions (e.g. as in LTE) reuse DMRS or CRS for this purpose.
In 5G system deployments at higher carrier frequencies, the radio link will exhibit some new properties compared to LTE at lower carrier frequencies. One of the fundamental changes is that the phase noise problem is scaled with frequency which introduces a need for a new phase reference signal to mitigate phase noise that is common for all subcarriers within an OFDM symbol. This reference signal may be needed both in uplink and downlink. It is foreseen that this signal can be used for both fine carrier frequency-synchronization and phase noise compensation. Where the second is the focus, the reference signal may be referred to as the Phase Noise Tracking Reference Signal (PNT-RS).
FIG. 2 illustrates an example time-frequency grids containing DMRS and PNT-RSs. The illustrated design is just one example since the design has not yet been specified in 3GPP. As depicted, the reference signal is transmitted time continuously and a length 8 cover-code is assumed to be used to create 8 orthogonal DMRS resources. The DMRS resource can be enumerated 0 . . . 7 and can be considered to be 8 DMRS ports. In the illustrated example, four different PNT-RS are depicted to support four transmitters with different phase noise.
The PNT-RS is transmitted jointly with the DMRS. As such, the PNT-RS is also transmitted jointly with the PDCH on a subset of the subcarriers that are used to transmit the DMRS. The DMRS is here assumed to be transmitted in one or a few OFDM symbols early within a subframe or within a subframe aggregation whereas the PNT-RS may be possibly transmitted in every OFDM symbol. The density of the DMRS in the frequency domain is significantly higher than the corresponding density of the PNT-RS. Thus, the set of subcarriers occupied by DMRS is significantly higher than the set of subcarriers occupied by the PNT-RS. In contrast to radio channels that are often non-flat over the transmission bandwidth, the phase ambiguity caused by the phase noise will impact all subcarriers in a similar way. The reasoning for transmitting PNT-RS on more than one subcarrier is basically then to obtain frequency diversity as well as increasing the processing gain.
Multi-layer transmissions of PDCH require a set of orthogonal DMRS which can be constructed by either interleaved Frequency Division Multiplexing (FDM) or Code Division Multiplexing (CDM) or both. Interleaved FDM or CDM may also be known as combs in LTE. CDM may refer to Orthogonal Cover Codes (OCC) based on, for example, Walsh-Hadamard codes or DFT or any other schemes that may provide orthogonality in code domain. In certain embodiments, OCC in time domain may be less suitable when phase noise needs to be tracked within the subframe. Moreover, DMRS needs to be transmitted in multiple OFDM symbols which may not always be the case in NX. Therefore, if sub-carriers are blocked by PNT-RS, the maximum number of available DMRS may be limited.
In a traditional synchronized radio system, such as, for example, LTE, some signals are always present to allow the UE to find the signals without having to communicate with the network first. Examples of such signals include Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and CRS. These types of signals allow the UE to keep time-frequency sync with the network. However, the always-on signals add some complexity to the radio system, result in bad energy performance, and provide constant interference.
Some more recent solutions include a lean system design that removes said signals. A problem with these designs is that the sync-procedure becomes more complicated and overhead has increased in terms of PNT-RSs using a large fraction of the spectrum at the cost of decreased data rates. For example, reusing DMRS is inefficient as the needed time-density is high for accurate phase noise tracking. The DMRS design takes frequency selectivity into account implying that the resource density in frequency needs to be rather high for demodulation performance. Thus, if the same signal is used for both demodulation and phase noise tracking, unnecessary high overhead may be created.
Accordingly, a separate PNT-RS may be used. However, this creates a problem in that the sub-carriers used for phase noise tracking are blocked as the OFDM symbols used for DMRS also contain phase noise and needs a phase noise reference. Thus, an early placement of a larger number of DMRS can be problematic if a large number of orthogonal DMRS are needed with early DMRS.